Is it possible to re render a single pass and not the whole scene again?
It appears that the answer is no, however I'm not sure why this wouldn't be possible for some passes such as Object Index, Material Index, etc.)

As a workaround, render only one sample (you can also render at a lower resolution while testing). 

To combine with the original render you will have to save the render to the hard drive (you may want to use exr to preserve other passes) and then load it back into the compositor with an Image node.